As the electronics industry progresses, a lot of fine droplets under submicron size have been required in various worked materials, ceramic materials and alloys. In particular particles of nanometer size which have a low sintering temperature and high reactivity owing to large surface area, have been widely applied in the preparation of advanced materials, since it eases the preparation of inexpensive highly qualified materials. Accordingly, studies on the development of an apparatus for producing ultrafine particles have been actively carried out, since ultrafine particles having a size of submicron or below are essentially required in the preparation of various worked materials, ceramic materials and alloys as mentioned aboves.
The ultrafine particles of nanometer size have been prepared by the following methods conventional in the art.
Fine droplets can be produced by an aerosol generator, and dried and pyrolyzed to give homogeneous fine particles of submicron size, by the spray pyrolysis method. According to the spray pyrolysis method, the size of the particles is controlled primarily by the concentration of the solution employed in the method, i.e., fine particles are prepared with a solution of low concentration, which, in turn, decreases productivity of the particles. In addition to this, the spray pyrolysis method has revealed some shortcomings that: ultrafine particles of nanometer size can be produced by the decomposition of large particles, only when the solution of some solutes causing highly exothermic reaction is used; ultrafine particles thus prepared are very cohesive; and, the apparatus employed therein is operated with a low reproducibility.
Fine droplets can also be prepared with a proper solution by an aerosol generator, which are deposited on a substrate to form thin layers employed in the preparation of semiconductor, etc. The said method has an advantage that a thin layer of multicomponents can be prepared with a high growth rate, when compared with the conventional CVD(chemical vapor deposition) method, while properties of the thin layer thus prepared depend largely on the characteristics of the droplets.
On the other hand, aerosol etching method in which fine droplets are sprayed by aerosol generator and accelerated under a low pressure to strike substrate being etched, has been suggested in the art. It possesses advantages of both the conventional dry-etching method and the wet-etching method in which chemical etching is carried out with an etching solution. However, needs have continued for the development of apparatus to generate fine droplets of small size, since the efficiency of the aerosol etching method depends on the size and distribution of droplets generated by an aerosol generator.
As a consequence, to accomplish the preparation of ultrafine particles and thin layers, and the efficient etching in the preparation of semiconductors, etc., more improved aerosol generators producing fine droplets massively have been suggested in the art, some of which are disclosed in the following references:
SU 1,407,570 describes an apparatus generating droplets employing a nozzle under a high pressure. The said apparatus has a simple structure and generates a large number of droplets, the size of the droplets thus generated, however, has a size larger than about 10 micron. Thus, it has not been used widely in the preparation of thin layer and ultrafine particles whose size is submicrons or below. Moreover, it has revealed some disadvantages in that high pressure is essentially required and the size of droplets can not be modulated.
Lang, R. J. reported an apparatus generating droplets by ultrasonic spray(see: Lang, R. J., Journal of Acoustical Society of America, 34(1):6-9(1962)). The aerosol generator can produce droplets of micron size to be used effectively in the preparation of ultrafine particles and thin layers, and aerosol etching, however the said prior art aerosol generators are proven to be less satisfactory in the sense that it can not produce a large number of droplets, and whose size can not be modulated, and it may be influenced by surface tension of the solution, etc.
On the other hand, Meesters, G. M. H. et al. teaches an apparatus which produces droplets of micron size by generating a strong electric field in a solution passing through a nozzle employing taylor cone(see: Meesters, G. M. H. et al., Journal of Aerosol Science, 23(1):37-49(1992)). However, the aerosol generator has revealed some shortcomings that it can not produce a large number of droplets, and whose size can not be modulated, while requiring substantial energy and high pressure.
In addition to the aerosol generators mentioned above, vibrating orifice and electrospray, etc. have been suggested in the art, however, they require high pressure, can not produce a large quantity of droplets, and have difficulty in modulating the size of droplets, which naturally limits their practical applications to industry.
Under the circumstances, there are strong reasons for exploring alternative apparatus producing a large quantity of ultrafine particles of submicron size under a lower pressure to prepare finer particles while the productivity is not decreased.